A mixture to be used in an absorption machine

ABSTRACT

There is disclosed an absorption machine comprising at least a first and a second compartment in fluid connection with each other, wherein the first compartment comprises at least one salt selected from the group consisting of LiBr, Lil, LiCl, Nal, and NH4I and wherein at least the first compartment comprises NH3 in an amount sufficient to form a liquid together with the at least one salt in the first compartment. Advantages of using the new mixture include that an absorption machine using a salt and NH3 can be made smaller and lighter with the same power. Further ΔT can be improved. The vapour pressure of NH3 in the system can be kept relatively high.

TECHNICAL FIELD

The present invention relates generally to a mixture to be used in an absorption machine such as a chemical heat pump as well as a chemical heat pump including such a mixture.

BACKGROUND

Generally chemical heat pumps and absorption machines are known. Such devices typically comprise an absorbing material or substance/mixture and a volatile liquid.

Lithium iodide is known for use in for instance batteries. There exist an anhydrous form as well as a monohydrate, a dihydrate, and a trihydrate of lithium iodide. Also other hygroscopic salts are known which form hydrates with water.

U.S. Pat. No. 3,312,077 discloses a lithium iodide-ammonia solution used as the absorbent and ammonia used as the refrigerant in an absorption refrigeration system. The system does not contain water according to the description.

Thus it is a problem how to improve the temperature difference ΔT utilized by a chemical heat pump. It is further a problem how to increase the power of a chemical heat pump as well as how to reduce its size and weight. In view of U.S. Pat. No. 3,312,077 it is further a problem how to increase the vapor pressure in the system.

SUMMARY

It is an object of the present invention to obviate at least some of the disadvantages in the prior art and provide an improved absorption machine.

In a first aspect there is provided an absorption machine comprising at least a first and a second compartment in fluid connection with each other, wherein the first compartment comprises at least one salt selected from the group consisting of LiBr, LiI, LiCl, NaI, and NH₄I and wherein at least the first compartment comprises NH₃ in an amount sufficient to form a liquid together with the at least one salt in the first compartment.

Further aspects and embodiments are defined in the appended claims, which are specifically incorporated herein by reference.

One advantage is that the efficiency of an absorption machine can be increased. ΔT can be increased. The machine can be made compact in relation to its power and/or energy storage capacity.

Another advantage is that the liquid matrix is simple and inexpensive to manufacture.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is now described, by way of example, with reference to the accompanying drawings, in which:

FIG. 1 shows the vapor pressure for LiI.3H₂O+NH₃ at different concentrations of NH₃ measured in M (mole/litre)

FIG. 2 shows a diagram from an experiment with discharging a system with LiI.3H₂O+NH₃.

FIG. 3 shows temperatures in an example without addition of graphene.

FIG. 4 shows the same temperatures of another example with addition of graphene.

FIG. 5 shows a schematic overview of the setup of an example.

DESCRIPTION

It is an object of the present invention to obviate at least some of the disadvantages in the prior art and provide an improved mixture for a chemical heat pump.

In a first aspect there is provided an absorption machine comprising at least a first and a second compartment in fluid connection with each other, wherein the first compartment comprises at least one salt selected from the group consisting of LiBr, LiI, LiCl, NaI, and NH₄I and wherein at least the first compartment comprises NH₃ in an amount sufficient to form a liquid together with the at least one salt in the first compartment.

In one embodiment the first compartment comprises LiI.3H₂O and wherein at least the first compartment comprises NH₃ in an amount sufficient to form a liquid together with the at least one salt in the first compartment.

The absorption machine according to any one of claims 1-2, wherein no water is present in the first compartment except water in an amount corresponding to hydrates of the at least one saltone salt. All water which is present or essentially all water, i.e. more than 99 wt % of the water in the machine is present in an amount corresponding to hydrate of the at least one salt. One example is LiI.3H₂O, where the amount of water in the system does not exceed the amount which can be present in the hydride. The amount of water in the system is not allowed to exceed the amount which can form hydrate with the at least one salt with more than 1 wt %. In one embodiment all water in the system corresponds to the maximum amount that can be present as hydrate of the at least one salt.

Normally the salt is present in the first compartment and the ammonia is absorbed by the salt to form a liquid in the first compartment. Ammonia can then be desorbed from the liquid in the first compartment during use of the machine. The amount of ammonia varies depending on the salt used and the temperature and pressure. In one example below the mixture of salt LiI.3H₂O and ammonia starts to become liquid when about 1.4 equivalents of ammonia is added, i.e. 1.4 molecules of NH₃ for each molecule of LiI.3H₂O.

In general it is desired to ensure that sufficient ammonia is added so that the solution is liquid and does not solidify. In one embodiment 2-5 equivalents of NH₃ for each molecule of salt (such as LiI.3H₂O) is used. In an alternative embodiment 3-4 equivalents of NH₃ for each molecule of salt is used. In yet another embodiment more than 2 equivalents of NH₃ for each molecule of salt is used. In still another embodiment more than 1.5 equivalents of NH₃ for each molecule of salt is used.

In one embodiment there is provided an absorption machine comprising at least a first and a second compartment in fluid connection with each other, wherein the first compartment comprises LiI.3H₂O and wherein at least the first compartment comprises NH₃ in an amount sufficient to form a liquid together with the at least one salt in the first compartment. This particular embodiment with LiI.3H₂O and NH₃ is further described in the examples section.

In one embodiment both the first and second compartments are in heat conducting connection with at least one surrounding system adapted to transfer heat to and from said first and second compartments. Thus the absorption machine can be used for various purposes including heating and cooling applications as well as heat transfer applications.

In one embodiment the pressure can be regulated in at least one of said first and second compartments. In yet another embodiment the pressure can be held higher in one of the at least two compartments compared to the other(s). It is conceived that the pressure regulation takes place with known methods such including but not limited to valves, pressure reducing valves and pumps.

In one embodiment the absorption machine is a chemical heat pump working according to the absorption principle, including: a reactor part comprising at least one salt such as LiI.3H₂O and arranged to be heated and cooled by an external medium, an evaporator/condenser part containing the portion of the NH₃ that exists in a condensed state, and arranged to be heated and cooled by an external medium, and a channel for the vapor phase of NH₃, the channel connecting the reactor part and the evaporator/condenser part to each other.

In one embodiment at least one of the first compartment and the second compartment comprises particles. In another embodiment at least one of the first compartment and the second compartment comprises particles with a maximum diameter in the range 1-100 nm. In yet another embodiment at least one of the first compartment and the second compartment comprises two dimensional particles. In one embodiment the particles are present in the first compartment only. Two dimensional particles include but are not limited to particles of graphene, which extends mainly in two dimensions with the third dimension being only one or a few atom layers. The two dimensional particles wherein the size in two dimensions is much larger compared to the thickness can be referred to as flakes. In one embodiment the thickness is less than 10⁻² or 10⁻³ of the lateral size, in further embodiments even less than 10⁻⁴ or 10⁻⁵. In a further embodiment at least one of the first compartment and the second compartment comprises particles comprising graphene. In one embodiment at least one of the first compartment and the second compartment comprises particles comprising graphene with a size in the interval 0.01-10 μm, preferably 0.1-1 μm. This size refers to the maximum distance in two dimensions, while the third dimension, the thickness is very thin, only one or a few atoms thick. In one embodiment the amount of graphene in relation so salt is 0.001 to 0.1 wt % calculated as the weight of graphene divided by the weight of the salt including the hydrate.

Advantages of using particles is that the heat conductivity is improved. The particles improve the heat transfer from the solution of the at least one salt and/or NH₃ to the wall enclosing the compartment. This effect is demonstrated in the example section. It can be seen that there is a notable improvement when graphene is added.

Further aspects and embodiments are defined in the appended claims.

One advantage is that an absorption machine such as a chemical heat pump using for instance LiI.3H₂O+NH₃ or another salt can be made smaller and lighter with the same power. Further ΔT can be improved. The vapor pressure of ammonia in the system can be kept relatively high. One advantage of using a system where a lot of the water is bound as hydrates to the salt, is that the partial pressure of water can be kept very low in the system allowing for efficient use of ammonia in the gas phase instead, which gives advantages for instance a better ΔT and the possibility to work at different temperatures compared to water. At the same time a liquid phase can be utilized which also gives advantages in terms of heat conduction etc. In view of this a skilled person realizes that if more water than the amount corresponding to the hydrate of the salt is added, then the function will gradually be less efficient when more water is added.

Before the invention is disclosed and described in detail, it is to be understood that this invention is not limited to particular compounds, configurations, method steps, substrates, and materials disclosed herein as such compounds, configurations, method steps, substrates, and materials may vary somewhat. It is also to be understood that the terminology employed herein is used for the purpose of describing particular embodiments only and is not intended to be limiting since the scope of the present invention is limited only by the appended claims and equivalents thereof.

It must be noted that, as used in this specification and the appended claims, the singular forms “a”, “an” and “the” include plural referents unless the context clearly dictates otherwise.

If nothing else is defined, any terms and scientific terminology used herein are intended to have the meanings commonly understood by those of skill in the art to which this invention pertains.

For absorption machines in general utilizing both NH₃ and water it is a problem that water follows with NH₃ when the mixture is heated and the water following the NH₃ constitutes a problem. In the present invention this is solved by using a salt such as LiI.3H₂O to which NH₃ is absorbed, wherein water is bound to the salt as a hydrate. The vapor pressure for LiI.3H₂O+NH₃ is depicted in FIG. 1 and as can be seen a considerable ΔT can be achieved. With 4 M of NH₃ the ΔT is close to 70° C. and with 1.77 M close to 120° C.

The solution of the salt such as LiI.3H₂O and NH₃ does comprise water, but in this environment water has such a low vapor pressure so that the amount of water following NH₃ during heating does not affect the performance to any notable degree. Further this water goes back together with NH₃ during discharging. Even if the water can be present both as a hydrate and to some limited extend as free water, the total amount of water does not exceed the amount of water that can be present in the form of hydrate of the at least one salt. Because of the properties of the at least one salt the partial pressure of water in the system will be kept very low.

Temperature stability tests have given that a temperature of 200° C. does not affect the stability of the system.

FIG. 2 discloses a diagram from a discharge where LiI.3H₂O is kept at about 60° C. while stirred with a magnetic stirrer while NH₃ is taken up from an insulated compartment. The NH₃ is a free fluid in the insulated compartment, i.e. a reactor. It can be seen that the cooling capability is high.

Since NH₃ is miscible with water it is conceived that some of the NH₃ may form a solution with accessible H₂O molecules from LiI.3H₂O.

EXAMPLES

In order to check for the presence of crystals a solution of LiI.3H₂O and NH₃ with 2.55 M NH₃ was put in a glass flask on an ice bath at ambient pressure. No crystallization could be detected. In another experiment 100 g of LiI.3H₂O was put in a flask and NH₃ was added to a pressure of 80 mbar corresponding to 1.77 M NH₃. The solution was cooled on ice, but no crystallization could be seen.

In order to investigate the stability, the following test was performed: 100 g LiI.3H₂O was mixed with 4 moles NH₃. This was heated during stirring in an evaporator connected to a condenser. The condenser was kept a room temperature (about 20° C.). The temperature in the evaporator was increased to 200° C. at a pressure of 6.3 bar. At this temperature and pressure a valve between the evaporator and the condenser was closed and the reactor was allowed to cool to room temperature. The condenser was cooled with ice to 0° C. and the pressure was 4.3 bar. NH₃ from the condenser was removed and replaced with new NH₃. This was also cooled with ice to 0° C. and the pressure was still 4.3 bar. This shows that the condenser after uptake of NH₃ from the condenser at 200° C. has not received any water.

Example 1

An experiment using the first compartment of the device, a reactor. The first compartment comprises LiI.3H₂O and NH₃. LiI.3H₂O is present as soft crystals in pure form. When mixed with NH₃ at room temperature NH₃ is absorbed by LiI.3H₂O. From experience it is known that when about 1.4 HN₃ molecule per LiI.3H₂O has been absorbed the salt starts to become liquid.

The example is not a complete absorption machine, but instead the reactor part (first compartment) is investigated in a model in order to study the heat transfer capability of the material. The setup is described schematically in FIG. 5 with an amount of liquid LiI.3H₂O and NH₃ (1), a space (2) above the liquid-gas interface, a heat exchanger (3), a spray nozzle (4), a pump (5) for the liquid LiI.3H₂O and NH₃, a thermometer (6), a pump (7) for a heat transferring medium, en electrical heater (8) and a thermometer (9) for the heat transferring medium.

The first compartment comprises a lithium iodide trihydrate ammoniate, formed when NH₃ is allowed to react with LiI.3H₂O. The compartment was filled with 1 kg lithium iodide trihydrate and it was allowed to absorb 3 molecules of ammonia per LiI.3H₂O. LiI.3H₂O was kept in a chamber where the ambient air was pumped out whereafter the desired volume of NH₃ was added. No additional water was added in addition to the crystal water in the trihydrate, i.e. the only water present corresponds to the amount that can be presen as a hydride in the salt. In this example, 3 ammonia molecules were absorbed per lithium iodide unit. This becomes a liquid under ambient pressure and room temperature. The liquid is pumped by the spray pump (5) to a spray nozzle (4) over a heat exchanger (3) secondary side. Heat was applied to the heat exchangers primary side from an electrical heater (8) via water circulated by a pump (7).

The temperature of the applied heat was measured with a first thermometer (9) at the primary side of the heat exchanger. The temperature of the lithium iodide trihydrate ammoniate was measured by a thermometer (6) in the flow from the spray pump to the spray nozzle.

In a first experiment the first compartment comprised lithium iodide trihydrate ammoniate. The spray pump and the circulation pump were started and the two temperatures were measured without electrical heating. The system was run in a room with an ambient temperature of 20° C. until the two temperatures were identical.

When the two different temperatures were equal and stable over time the electrical heating was switched on. The resulting temperatures can be seen in FIG. 3. It can be seen that the temperature difference after 3 minutes is about 5° C. and after 9 minutes it is about 7° C. It can be seen that the transfer of heat is impaired with increasing temperature.

Example 2

The experiment was repeated with a different content in the compartment. Graphene was added to the system in order to investigate its influence on the heat transfer properties.

Graphene was added in a suspension in water to lithium iodide trihydrate. The concentration of graphene in the suspension in water was 0.2 wt %. The graphene was in the form of thin flakes with a size in the interval 0.1-1 μm. 1 kg lithium iodide trihydrate was used. Then the water solution comprising graphene was evaporated until lithium iodide trihydrate remained. Water was evaporated and the weight was measured so that the weight corresponded to lithium iodide trihydrade and graphene. Thus all added water was removed. This mixture was allowed to absorb 3 molecules of ammonia per LiI.3H₂O. LiI.3H₂O was kept in a chamber where the ambient air was pumped out whereafter the desired volume of NH₃ was added. Only an amount of water corresponding to the hydrate was left.

The experiment in example 1 was repeated with temperature stabilization and heating.

In FIG. 4 it can be seen that the temperature difference after 3 minutes is still about 5° C. but after 9 minutes it is still about 5° C. It can be concluded that graphene has affected the heat transfer of the material in a positive way at elevated temperatures.

It is envisaged that the temperature of the system in many applications normally will be above 70° C., where graphene has a positive influence.

From these data it is estimated that graphene can improve the heat transfer capability of the material by about 30%.

All the described alternative embodiments above or parts of an embodiment can be freely combined without departing from the inventive idea as long as the combination is not contradictory.

Other features and uses of the invention and their associated advantages will be evident to a person skilled in the art upon reading the description and the examples.

It is to be understood that this invention is not limited to the particular embodiments shown here. The embodiments are provided for illustrative purposes and are not intended to limit the scope of the invention since the scope of the present invention is limited only by the appended claims and equivalents thereof. 

1.-12. (canceled)
 13. An absorption machine comprising at least a first and a second compartment in fluid connection with each other, wherein the first compartment comprises at least one salt selected from the group consisting of LiBr, LiI, LiCl, NaI, and NH₄I and wherein at least the first compartment comprises NH₃ in an amount sufficient to form a liquid together with the at least one salt in the first compartment.
 14. The absorption machine according to claim 13, wherein the first compartment comprises LiI.3H₂O.
 15. The absorption machine according to claim 13, wherein no water is present in the first compartment except water in an amount corresponding to hydrates of the at least one salt.
 16. The absorption machine according to claim 13, wherein both the first and second compartments are in heat conducting connection with at least one surrounding system adapted to transfer heat to and from said first and second compartments.
 17. The absorption machine according to claim 13, wherein the pressure can be regulated in at least one of said first and second compartments.
 18. The absorption machine according to claim 13, wherein the pressure can be held higher in one of the at least two compartments compared to the other(s).
 19. The absorption machine according to claim 13, wherein the absorption machine is a chemical heat pump working according to the absorption principle, including: a reactor part comprising the at least one salt and arranged to be heated and cooled by an external medium, an evaporator/condenser part containing the portion of the NH₃ that exists in a condensed state, and arranged to be heated and cooled by an external medium, and a channel for the vapour phase of NH₃, the channel connecting the reactor part and the evaporator/condenser part to each other.
 20. The absorption machine according to claim 13, wherein at least one of the first compartment and the second compartment comprises particles.
 21. The absorption machine according to claim 13, wherein at least one of the first compartment and the second compartment comprises particles with a maximum diameter in the range 1-100 nm.
 22. The absorption machine according to claim 13, wherein at least one of the first compartment and the second compartment comprises two dimensional particles.
 23. The absorption machine according to claim 13, wherein at least one of the first compartment and the second compartment comprises particles comprising graphene.
 24. The absorption machine according to claim 13, wherein at least one of the first compartment and the second compartment comprises particles comprising graphene with a size in the interval 0.01-10 μm.
 25. The absorption machine according to claim 13, wherein at least one of the first compartment and the second compartment comprises particles comprising graphene with a size in the interval 0.1-1 μm. 